Toner Functions
In order to develop the intended image in an electrophotographic hardcopy device, liquid toners must perform the following functions:
a. Have color, including black; PA1 b. Form a stable dispersion in the carrier liquid, or else form an intentionally unstable dispersion that is readily re-dispersed on demand in the EP hardcopy device; PA1 c. Possess electrical charge on a permanent or at least semipermanent basis, in order to respond to the imposed electrical bias which is the driving force in this imaging technique; and, PA1 d. Have a fusible resin coating layer that imparts permanence and durability to the hardcopy image.
Prior Toner Synthesis
The prior art teaches the above toner functions a-d, but shows them to be performed by several different parts of the toner particle. The synthesis of these prior toners involves many steps and is complex. These toner-synthesis processes do not lend themselves well to continuous manufacturing processes and are often performed in batch mode. This introduces batch-to-batch variation, and increases the need for careful quality control.
The following are typical of the many patents teaching this multicomponent kind of toner: Gibson et al., U.S. Pat. No. 4,897,332; Elmasry et al., U.S. Pat. No. 4,925,766; Swidler. and U.S. Pat. No. 5,045,425. Usually a dye or pigment provides the colorant material for the toner particle. A polymeric resin is bound to the colorant via specific or nonspecific adsorption to the surface of the colorant. A charge director is added, and sometimes charge enhancers are also added to assist in binding the charge to the particle. There may be additional surfactants and release agents to stabilize the dispersion of toner particles in the carrier liquid or to promote transfer of the developed image to the receiving paper or other medium.
Besides being multicomponent systems, these prior toners are highly polydisperse in terms of particle identity, particle size and electrophoretic mobility. A toner dispersion of this type typically contains free (uncoated) pigment chunks, resin-coated pigment chunks, free resin, free charge director, and aggregates of all sizes up to 100 microns or more in diameter. This polydispersity impacts negatively on dispersion stability, shelf life, print quality, and even the ability to characterize the dispersions with off-line analytical techniques.
Dendritic Polymers
A review of the synthesis, characterization and applications of dendritic polymers ("dendrimers") reveals that they have been known since 1978. Dendrimer chemistry is discussed in general in "Starbursts.TM./Cascade Dendrimers: Fundamental Building Blocks for a New Nanoscopic Chemistry Set", by Donald A. Tomalia, Aldrichimica Acta, Vol. 26, No. 4, 1993, pages 91-101. Dendrimers are monodisperse enough to be used as size calibration standards. (D. A. Tomalia, L. R. Wilson, European Patent Application 247629 A2, Dec. 2, 1987; P. L. Dubin, et al. Anal. Chem., 1992, 64(20) 23344-7.) One-step syntheses of dendrimers have been reported. (C. J. Hawker, R. Lee, J. M. J. Frechet, J. Amer. Chem. Soc., 1191,113(12).; T. M. Miller, et al. J. Amer. Chem Soc., 1993,115(1) 356-7.) The comb-burst dendritic topology permits dendritic grafting onto other (non-dendritic) molecules. (D. A. Tomalia, et al. Macromolecules, 1991, 24(6) pp 1435-8.) Size, shape, surface chemistry, topology and flexibility of dendrimers are all controllable parameters. (D. A. Tomalia, D. M. Hedstrand, Actual. Chim. 1992,5(347).) Glass transition temperature of dendrimers is also controllable. (K. L. Wooley, et al. Macromolecules, 1993, 26(7)1514-19.)
Dendrimers have been applied in imaging technologies other than electrophotography, such as jet-printing inks and medical imaging. Winnik. et al., U.S. Pat. No. 5,098,475, "Inks with Dendrimer Colorants" and Winnik. et al., U.S. Pat. No. 5,120,361, "Ink Compositions" disclose aqueous ink compositions with a dendrimer that is colored with a dye or dyes covalently attached thereto and ink compositions comprising a dendrimer and a dye or dyes, respectfully. Watson, PCT International Application, WO 93/06868 A1, "Dendrimeric Polychelants" discloses dendrimeric polychelants for medical imaging, "comprising an up to fifth generation dendrimeric backbone moiety with linked thereto a plurality of macrocyclic moieties capable of complexing metal ions, and metal chelates and salts thereof".
Dendrimers have been disclosed for use as charge-enhancing additives for electrophotography toners. Duff, et al., U.S. Pat. No. 5,332,640 discloses multicomponent toner compositions comprising resin particles, pigment particles, and dendrimeric charge enhancing additives, wherein these toner components are combined by such methods as dry blending techniques, extrusion melt blending, or encapsulation.